The electrolysis of alkali metal halide brines, such as sodium chloride and potassium chloride brines, in electrolysis cells is a well known commercial process. Electrolysis of such brines results in the production of halogen, hydrogen and aqueous metal hydroxide. In the case of sodium chloride brines, the halogen produced is chlorine and the alkali metal hydroxide is sodium hydroxide. The electrolytic cell typically comprises an anolyte compartment containing an anode, and a separate catholyte compartment containing a cathode assembly. The cathode assembly is typically comprised of a cathode and a liquid-permeable diaphragm, which partitions the electrolytic cell into the anolyte and catholyte compartments.
For the cell to operate properly, it is required that the diaphragm be sufficiently porous to allow the hydrodynamic flow of brine through it, while at the same time inhibiting the back migration of hydroxyl ions from the catholyte compartment into the anolyte compartment. The diaphragm should also (a) inhibit the mixing of evolved hydrogen and chlorine gases, and (b) possess low electrical resistance, i.e., have a low IR drop. Historically, asbestos has been the most common diaphragm material used in these chlor-alkali electrolytic diaphragm cells. Subsequently, asbestos in combination with various polymeric resins, particularly fluoro-carbon resins (the so-called polymer-modified asbestos diaphragms), have been used as diaphragm materials.
Owing to the health and environmental concerns associated with airborne asbestos fibers, the development of asbestos-free diaphragms for use in chlor-alkali electrolytic cells has been an area of ongoing exploration. Such diaphragms, which are often referred to as synthetic diaphragms, are typically fabricated from non-asbestos fibrous polymeric materials that are resistant to the corrosive environment of the operating chlor-alkali cell. These materials are typically perfluorinated polymeric materials, e.g., polytetrafluoroethylene (PTFE). The synthetic diaphragms may also contain various other modifiers and additives, including inorganic fillers, pore formers, wetting agents, ion-exchange resins and the like.
It is known that synthetic diaphragms for chlor-alkali cells can be prepared by coating and/or impregnating them with inorganic materials. However, such procedures generally require the addition of inorganic material or coating to a pre-formed diaphragm, thus requiring additional processing, equipment, and adding labor and cost.
It would be desirable, then, to develop a porous, non-asbestos separator and a method of forming the separator which provides the ability to control the separator's permeability, pore size, and tortuosity, thereby providing improved uniformity, operating efficiencies, and reduced energy consumption, in chlor-alkali cells without the necessity for additional equipment or processing steps.